This invention relates generally to modems and communication systems for the transmission of data over a switched network. More particularly, this invention relates to an intelligent synchronous modem and a communication system incorporating an intelligent synchronous modem which will permit the interface of a microcomputer or personal computer with a mainframe computer requiring synchronous communication.
Both synchronous modems and intelligent asynchronous modems are known in the art. Typically, database services utilize mainframes which communicate in an asynchronous mode. Thus, with an intelligent asynchronous modem, a user could insert a disk with communication software into his PC and start the program. The software, in command mode, then instructs the modem to dial the telephone number keyed in by the user with the command: ATD (xxx) xxx-xxxx; where "AT" means "attention", and "D" means "dial the following number". The modem will then dial the given number and interrupt the PC with the message "CONNECT" when the connection with the remote computer has been accomplished. The modem will then leave its "command mode" and enter its "data mode". Once in the data mode, the modem simply acts as a modulator/demodulator for whatever information is being transmitted between the computers. After data transmission is completed, the computer will then use command mode to instruct the modem to "hang up". Because the modem can understand when it is being commanded to enter the command and data modes, it is called an "intelligent" modem. Those skilled in the art will understand that the nature of asynchronous communication permits the implementation of such an intelligent modem, as asynchronous data is sent at whatever rate it is received, and it is not enveloped with excess information which must be stripped to understand the data. Thus, the appearance of particular control language, in any location and at any time in the data stream, will cause the intelligent modem to enter or leave command mode.
Often, in large corporations, computer systems are arranged for synchronous communication, and synchronous modems have long been available to facilitate transmission of synchronous messages. Synchronous data is sent according to a particular protocol and according to a clock. Because a protocol is used, a PC must be arranged to talk according to that protocol. Typically, a "SYNC card" is used to generate the proper protocol and thus translate the information being output by the PC into properly formatted information. The information may then be sent via the modem to the mainframe computer. The modem's function in such an arrangement is simply to convert each bit of information into phone line compatible information. Because the information is being sent according to a complex protocol, the modem (without the use of expensive and sophisticated hardware) is incapable of determining whether any of the information is meant for it rather than the distant computer. Thus, the synchronous modems of the art are non-intelligent. They act in "data mode" only and simply pump the information through the line. Typically, all "command mode" information is accomplished manually such that operators place the calls to the mainframe and manually transfer the line to the modem of the PC.